The present invention relates to signal sources for testing electronic equipment, and more particularly to amplitude flatness and phase linearity calibration for radio frequency (RF) signal sources.
A vector signal generator may be constructed using an arbitrary waveform generator (AWG) capable of wideband signal generation followed by one or more stages of block up-conversion and filtering. The up-conversion and filtering process may create significant amounts of amplitude ripple and deviation from linear phase across the frequency bandwidth of interest for the desired wideband test signal. These phase and magnitude effects are due to both the action of the filters and of the mixers themselves.
The frequency response of the up-conversion process may be corrected by applying a baseband correction filter having a response which, when cascaded with the up-converter, provides a flat amplitude and linear phase over the frequency bandwidth of interest. It is necessary to measure the frequency response characteristics of the up-converter in order to create the baseband correction filter. This measurement typically is done with external test equipment in a factory environment. However such measurement is inherently limited because the frequency response is subject to changes with temperature and age of components.
One method for measuring magnitude and phase of an instrument or transmission system using a power detector is described by Vasudev et al in IEEE Transactions on Microwave Theory and Techniques, Vol. 50, No. 9, September 2002, entitled “Measurement of a Filter Using a Power Detector.” This method describes measuring the phase at each end frequency which encompasses the frequency bandwidth of interest, and then measuring the phase at the midpoint between the two frequencies, then at the midpoint of two frequencies for which phase has already been measured, etc., until sufficient points have been measured to provide sufficient detail to describe the phase characteristics of the filter being measured across the frequency bandwidth, i.e., a “web algorithm.” Measurements are made at each frequency independently to obtain known phases at each frequency.
U.S. Pat. No. 8,224,269, issued Jul. 17, 2012 to Jungerman et al entitled “Vector Modulator Calibration System”, also uses a power sensor, such as a diode detector, to measure the power output from a vector modulator, i.e., the I and Q components of a vector signal which represents a single frequency signal with two components that are ninety degrees phase different.
What is desired is a means for measuring the phase and amplitude response of the RF signal source at the time of use in order to provide filter coefficients which are used to compensate for amplitude ripples and phase variations.